If the transistors are overkill, why do they fail so often?

From what I have read, this commonly happens if you hold the flipper up for too long. Since the flippers use PWM for hold power, and this a FET type transistor, I suspect that the transistor is dissipating a lot of heat from operating too much in the switching state versus full on or full off. The current rating only applies for transistors operating in the fully on state.

One way to test this I guess would be to hold the flipper up and put your finger on the transistor. If it is getting hot, that would be an indicator that these are failing not from too much current but rather from exceeding the junction temperature capability of the transistor. And if that is the case, a small heat sink would probably solve the problem.